Publisher Summary The chapter summarizes advances in interleukin-5 (IL-5) and IL-soluble receptor (IL-SR) research in the study of structure, physiologic functions, and the unique modes of receptor-mediated signaling. The chapter also discusses the pathophysiology of aberrant expression of IL-5 and its receptor. Helper T cells recognize antigenic peptide in the context of class IL1 major histocompatibility complex (MHC) molecules on accessory cells and/or B cells and secrete several soluble factors, including IL-4, IL-5, and IL-6, that can induce B-cell growth and the maturation of B cells. Mouse IL-5 (mIL-5) is a glycoprotein induced in T cells after stimulation with an antigen, such as Mycobacterium tuberculosis or Toxocara canis, and in mast cells upon stimulation with allergen/IgE complex or calcium ionophores. The action of IL-5 in responsive cells is the molecular mechanism of signal transduction cascade after IL-5 binding to the functional IL-5 receptor (IL-5R). The IL-5 signals can be transduced through the high-affinity IL-5R that consists of two different polypeptide subunits: α and β. The cDNAs encoding both α - and β -subunits for mIL-5R have been isolated; the α -subunit was found to be a 60 kDa (p60) protein and the β-subunit (a 130-kDa protein, p130) was identified as the protein identical to the β-subunit for mIL-3R and mGM-CSFR, which can convert the low affinity mILSRα into a high-affinity receptor. The study of mIL-5 originated from the search for the B-cell differentiation factor that induces antigen-primed B cells to differentiate into antigen-specific antibody-producing cells or the proliferation of BCLl B-cell tumor cells.